It is well known that very high contrast photographic images can be formed using certain types of silver halides, and methods for forming such photographic images are employed in the field of photomechanical process.
To attain this objective, a special developer called "a lith developer" has been used. The lith developer contains hydroquinone alone as a developing agent, and contains a sulfite which functions as a preservative in the form of the formaldehyde adduct. The free sulfite ion concentration is thereby reduced to about less than 0.1 mol/liter such that the infectious developability characteristic of hydroquinone is not inhibited by sulfite ion. Consequently, the lith developer is extremely prone to air oxidation, such that it cannot be stored for longer than 3 days.
Systems for obtaining the high contrast photographic characteristic by the use of a stable developer and a hydrazine derivative have been proposed, e.g., in U.S. Pat. Nos. 4,224,401, 4,168,977, 4,166,742, 4,311,781, 4,272,606, 4,211,857 and 4,243,739. According to these methods, extremely high contrast and high sensitivity characteristics are achieved and, additionally, since sulfite may be added to the developer in a high concentration, the stability of the developer to air oxidation is considerably enhanced as compared to the lith developer.
However, the above described methods of forming very high contrast images by the use of a hydrazine compound disadvantageously promote infectious development to a great extent. Thus, when photographing line originals of low contrast (particularly, fine line originals with Ming type characters), portions of the white background become blackened such that individual characters become solid black marks that are difficult to read. On the other hand, if the exposure is reduced to provide better reproduction of the fine lines of Ming type characters, Gothic type letters become distorted. Namely, a problem of the above described methods employing a hydrazine compound is the narrowness of the exposure latitude. A similar problem also occurs when photographing halftone dot images. Specifically, even partially white-on-black dots tend to be blackened characteristic of a narrow screen range, such that the above described methods are disadvantageous with respect to image quality. This is because the infectious developing action of the hydrazine compound in contrast development is so strong that even the less exposed or unexposed areas adjacent to the exposed areas are developed. To prevent this phenomenon, it has been desired to develop a method for suppressing the image expansion resulting from infectious development to thereby inhibit the development in portions adjacent to image portions (hereinafter called "microscopic development inhibition").
Moreover, an original used in the line drawing photographing step is prepared by combining photocomposed letters, handwritten letters, illustrations, halftone photographs and the like, such that the original contains a mixture of images differing in density and line width. Under these circumstances, the development of process cameras, photographic light-sensitive materials and image forming methods for use in duplicating line originals with good reproducibility has been strongly desired. In the photomechanical process for the preparation of catalogs and large sized posters, on the other hand, enlargement of halftone photographs ("spread") or reduction of halftone photographs ("choke") is generally carried out. Since lines are sparingly present in the photomechanical process using dots in an enlarging condition, photographs of blurred dots are taken. In the case of the reduction, the number of lines per inch becomes greater than that of the original, such that halftone photographs of the smaller dot areas are taken. Accordingly, an image forming method which provides a much wider latitude than conventional methods has been desired for attaining good reproducibility of the entire screen range.
While the inhibition of infectious development can be attained by reducing the addition amount of the nucleating agent or by lowering the pH of the developer, these means render the gradation soft to thereby reduce the image line sharpness. Therefore, such means for inhibiting the infectious development are not practical. In the system of nucleation development, various means for imparting the microscopic development inhibiting effect have been examined. However, no satisfactory means has yet been found.
In the present invention, the control of the nucleation development is carried out by adjusting the pH of the developer to 11.2 or lower. Although a sufficient increase in contrast is normally not brought about by lowering the pH 11.2 or lower, the lowering of the development pH together with the use of a nucleation accelerator can impart satisfactory contrast to the gradation. Moreover, the present inventors have discovered that the development carried out at a pH of 11.2 or lower inhibits the infectious development such that image expansion is small as compared with development at a higher pH.
On the other hand, the development reaction in the image area is generally attended by the release of hydrogen and halogen ions. The lowering of pH due to diffusion of hydrogen ion into the area adjacent to an image, and the diffusion of halogen ions thereinto cause microscopic development inhibition in the adjacent area. It has been also found out that these phenomena are liable to occur in the nucleation development carried out at a pH of 11.2 or lower.
In the nucleation systems utilizing hydrazine compounds as disclosed in JP-A-53-66732 (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application"), the addition of benzotriazoles to a developer is essential for the attainment of an increase in contrast without collateral generation of fog. However, in a high contrast development system having a pH of 11.2 or lower, the present inventors have discovered that an increase in contrast without collateral generation of fog occurs even in the absence of benzotriazoles. Furthermore, the microscopic inhibitor activity as described has been found to increase remarkably under these conditions. Although JP-A-53-66731 describes an increase in contrast attained by the use of a developer free from benzotriazoles and adjusted to pH 11.5 or higher, fog tends to be generated and the exposure latitude of such a system is narrow.
The improvement of image quality provided by the present invention, including enhancement of reproducibility of image lines and dots in photographing with a camera, results from the achievement of the present inventors in increasing contrast through nucleation at a development pH of 11.2 or lower, such that the addition of benzotriazoles to the processing solution is unnecessary. The absence of benzotriazoles from the developer is advantageous because the Br.crclbar. and pH dependencies in the developer become great and microscopic development inhibition becomes liable to occur.